1. Field of the Invention
The invention relates to a multielectrode spark plug which has improved resistance to fouling.
2. Description of the Related Art
A multielectrode spark plug is often used to reduce spark wear of a ground electrode and improve ignitability. A creeping spark plug or a semi-creeping spark plug is used to conduct burn-cleaning of conductive materials (mainly carbon caused by unburned fuel) deposited on the surface of the front end portion of an insulator and prevent fouling resistance from being impaired. As an example of such a spark plug, Japanese Patent Publication (Kokai) No. SHO51-95540 discloses a multigap spark plug having a plurality of ground electrodes 102, which are opposed to a center electrode 101, as shown in FIGS. 14A and 14B. The spark plug has two kinds of spark discharging gaps, namely, a semi-creeping spark discharge gap (creeping spark discharge gap 111+first aerial spark discharging gap 113), which is located partly along a tip end face of a front end portion of an insulator 104, and a second aerial spark discharging gap 112.
U.S. Pat. No. 2,650,583 discloses a spark plug 200 which, as shown in FIGS. 15A and 15B, has a plurality of layer-like ground electrodes 203 including electrodes 202, the tips of which oppose a center electrode 201, and has a plurality of spark discharging gaps formed between the center electrode 201 and the tips of the ground electrodes 203. The ground electrodes 203 of the spark plug 200 cover a part of a front face 205 of an insulator 204.
Furthermore, noble metal spark plugs which have noble metal fixed to a firing position of an electrode are popularly used to prevent spark wear, there by lengthening the life of the noble metal spark plug.
When a conventional spark plug 300 of the parallel electrode type, as shown in FIG. 16, is used in reversed polarity, a discharge voltage is raised. As a result, when smolder occurs, a discharge may not take place across the normal spark discharge gap. Specifically, when smolder occurs and the insulation resistance between a center electrode 301 and a ground electrode 302 is decreased, the output voltage of a power coil is divided by the output impedance of the power coil and the insulation resistance between the center electrode 301 and the ground electrode 302; hence, the voltage from the power coil, which appears across the normal spark discharge gap, is lowered. When smolder occurs and carbon is deposited, therefore, the discharge voltage at the normal spark discharge gap is raised and a discharge hardly takes place.
In the spark plug 100 disclosed in Japanese Patent Publication (Kokai) No. SHO51-95540 shown in FIGS. 14A and 14B, a spark discharge at the aerial spark discharge gap 112 occurs not largely far from a spark discharge by the creeping spark discharge gap (111+113). Specifically, when a spark discharge takes place across the creeping spark discharge gap, the spark is produced along the tip end face of the front end portion 106 of the insulator 104 and at the shortest distance between the front end portion 106 of the insulator 104 and the ground electrode 102. By contrast, when a spark discharge takes place across the second aerial spark discharge gap 112, the spark is produced at the shortest distance between the ground electrode 102 and the center electrode 101. The locations of the sparks in the air gaps differ substantially corresponding to the thickness of the ground electrode 102. Therefore, the location where a spark discharge takes place cannot be protruded from the front end of the spark plug and its ignitability cannot be sufficiently improved.
In the spark plug 200 of U.S. Pat. No. 2,650,583 shown in FIGS. 15A and 15B, a part of each ground electrode 203 partly covers the tip end face of the insulator 204. Therefore, it is impossible to conduct burn-cleaning of carbon deposited on the portions of the insulator 204 covered with the ground electrodes 203, and the ability of burning off carbon-adhering on the surface of the insulator 204 is decreased. When the distance between the front end portion of the insulator 204 and the ground electrodes 203 is short, a carbon bridge is easily produced, creating a high possibility of engine stall. When the distance between the front end portion of the insulator 204 and the ground electrodes 203 is long, a voltage required for producing a spark across the semi-creeping spark discharge gap is raised. Consequently, a spark takes place less likely along the front end portion of the insulator 204, and the cleaning ability of burning-off carbon deposited on the front end portion of the insulator 204 is lowered.